Slash and burn in the Amazon rainforest

Slash & burn farming is a traditional, nomadic form of farming of both tropical and temperate zones. The thin acidic rainforest soils are unsuited to cultivation but the acid is neutralised by the ash from burning, which thereby improves the soil. The method is suited to the tropical climate but the soil is exhausted in a few years and the farmers then must move on to burn another stretch of forest. The expansion of cash crops has caused excessive slash & burn farming in recent times, exceeding the power of the rainforest to recover and leading to encroaching desertification.
The Amazon rainforest has been declining rapidly since the early 20th century under pressure from felling and farming.